Resistance of tumor cells to cancer therapy, limited efficacy of cancer therapy in metastatic disease, and undesired host toxicity of cancer therapy are three significant challenges in patient management.
A common resistance mechanism to chemotherapy observed in preclinical studies is the overexpression of drug efflux proteins (Lum, B. L. et al. (1993) Cancer 72, 3502-3514; Barrand, M. A. et al. (1997) Gen. Pharmacol. 28, 639-645; Fidler, I. J. (1999) Cancer Chemother. Pharmacol. 43:S3-S10.). However, at least some clinical studies show that inhibition of the drug efflux proteins does not significantly improve the effectiveness of chemotherapy in patients (Ferry, D. R., et al. (1996) Eur. J. Cancer 32:1070-1081; Broxterman, H. J., et al. (1996) Eur. J. Cancer. 32:1024-1033), suggesting the existence of other resistance mechanisms.
Cancer therapy, such as chemotherapy and radiation, targets proliferating cells and thereby causes undesired toxicity to normal host tissues that undergo continuous renewal, including the hematopoietic cells, cells in the lining of the gastrointestinal tract, and hair follicles. Bone marrow suppression induced by cancer therapy is, at least in part, overcome by the use of hematopoietic growth factors, including erythropoietin, granulocytes colony-stimulating factor, and granulocyte-macrophage colony-stimulating factor (Gabrilove, J. L. and Goldie, D. W. (1993) In: Cancer, Principles & Practice of Oncology (eds. DeVita, V. T. et al., J. B. Lippincott Co., Philadelphia). On the other hand, no treatment is available to overcome the gastrointestinal toxicity and alopecia induced by anticancer agents.
Therefore, there exists a need to identify new mechanisms by which tumor and normal cells elude cytotoxicity of anticancer agents, to identify methods and agents to overcome such resistance in tumor cells, and to utilize these resistance mechanisms to protect normal host tissues from the undesired toxicity of cancer therapy.